Earth’s Longer Iron Age

The deep ocean remained iron-rich and devoid of oxygen far longer than previously thought, analysis of mineral deposits such as Mt Isa suggests.

Early in its existence the Earth’s atmosphere and oceans were devoid of oxygen. The oceans instead had high concentrations of iron.

But 2.4 billion years ago the planet experienced the Great Oxidation Event, when oxygen levels in the atmosphere rose dramatically. While still well below modern levels, this oxygen was sufficient to infiltrate the upper levels of the ocean, oxidising the iron and transforming these waters to something more like what we know today.

However, Dr Peter McGoldrick of the University of Tasmania has co-authored an online Nature paper suggesting it took almost two billion years before waters more than 100–200 metres deep were similarly oxidated.

“We knew from investigations into sedimentary zinc deposits undertaken a decade ago that Australian rocks 1.8–0.8 billion years old were unusually rich in iron,” McGoldrick says. His studies on these rocks and similar deposits in China and the US suggest they were formed from waters lacking oxygen.

McGoldrick’s co-author, Prof Tim Lyons of the University of California at Riverside, says the work suggests “a need to rethink all the models of how life-essential nutrients were distributed in the oceans through time and space”.

McGoldrick adds: “There are other implications for how giant zinc and lead ore deposits formed during Earth’s ‘middle age’, and where they might be found in sedimentary basins from this time”.

It is unclear what would have caused the oxygenation of the deep oceans after so long, but McGoldrick says the timing appears to coincide with the end of the events known as “snowball Earth” (AS, Jan/Feb 2011, p.44). McGoldrick says one theory has a burst of phosphorus being washed into the oceans as the ice retreated, spurring increased photosynthesis in the upper layers.

The conclusions suggest that mineral deposits of the era should have formed where the deep oceans met shallower shelves, causing the iron to oxidise. McGoldrick says “there are many ways to look for ore,” and doubts his work will change prospecting strategies.